ERROR: This account already exists on your device is displayed and you cannot sign in at Google Assistant setting.

You cannot sign in even when you select Sign in at Google Assistant setting during initial setup.

If you are already signed in with a Google account during initial setup, select Skip at Google Assistant setting.

After initial setup is finished, press the MIC/Assistant button on the remote control or select the MIC/Assistant icon on the Home screen. Google Assistant setting is displayed again, and you can finish setup without errors.